1. Field of the Invention
This invention relates to devices for warning a driver when his or her vehicle is approaching an obstacle (or a person), particularly but not necessarily only when backing up.
2. Description of the Prior Art
Devices of the general type are fairly well known. A typical prior art device uses an ultrasonic transmitter and an ultrasonic receiver, and triggers an alarm when a reflected signal from the transmitter of sufficient intensity is received by the receiver. The alarm may be audible, or visual, or both. Devices of this type are known from U.S. Pat. No. 5,574,426 (Shisgal et al.), U.S. Pat. No. 5,650,765 (Park), U.S. Pat. No. 5,764,136 (Harron), U.S. Pat. No. 4,903,004 (Starke et al.), U.S. Pat. No. 3,922,629 (Hayakawa) and U.S. Pat. No. 4,551,722 (Tsuda et al.).
A particular problem with at least some prior art devices is that if they become dirty, snow-covered, or otherwise inoperative, there may be no warning to the driver. The driver may therefore assume that backing up is safe, when in fact there could be an obstacle--or a child--in close proximity to the vehicle.
There is therefore a need for a device which alerts the driver when it is not functioning properly. It would also be desirable for the device to be configured in such a way as to remain as clean and un-obscured as possible, so that instances of inoperability are minimized.
A further problem is the transmission of ultrasonic vibration from the ultrasonic transmitter to adjacent parts of the vehicle. These adjacent parts may then transmit ultrasonic signals of their own, which interferes with the reading of bouncing echoes from the ultrasonic signals transmitted by the transmitter. To minimize this problem, it is known to decrease the sensitivity of the receiver, but this is disadvantageous when it comes to dirt or snow covered transmitters due to the lowered output signal.